Exhaust hoods are used to remove air contaminants close to the source of generation located in a conditioned space. For example, one type of exhaust hoods, kitchen range hoods, creates suction zones directly above ranges, fryers, or other sources of air contamination. The exhaust stream from such applications often contain large quantities of particulates, particularly hydrocarbons such as oil droplets. Organic substances in the form of vapours or particles can also be formed by many production processes within various industries. For example, they can be generated by preparation and use of lacquer and paint, cereal and feedstuff, metal and plastic, tar and asphalt, tanneries, incinerating plants, bio-gas plants, agriculture, and many food preparation processes.
Because of concerns about the environment and worker health, there is a great need for economically attractive mechanisms for removing organic substances from air streams. Air purification is frequently performed by filtering the contaminated air in, for example, grease filters and carbon filters. Mechanical filters, however, are expensive in terms of maintenance manpower and pressure drop (which leads to high operating costs. Furthermore, filters cannot guarantee fulfillment of high hygienic requirements.
One technology that has been used for degrading organic particulates in effluent streams is the addition of ozone to the effluent stream. This can be accomplished by irradiating with ultraviolet light or using a corona discharge. A negative side effect of using corona discharge is the creation of NOx.
One example of an application of ultraviolet light to the purification of an effluent stream is described in U.S. Pat. No. 6,179,969 to Larson. In the embodiment described, contaminated air flows into a chamber which and diluted with ambient air to cool it and add oxygen to the air stream. The effluent stream is then irradiated with ultraviolet light and then ejected to the ambient. The system may include a filter for removing larger particles before introduction of the effluent stream into the dilution section. Cooling of the stream causes condensation of water and certain organic vapors. The oxygen injected in the dilution process is used in the creation of ozone when the air is irradiated with the ultraviolet light. Certain organic substances are oxidized by the ozone in a so-called cold combustion and are thereby transformed into carbon dioxide, but many organic molecules can not be oxidized in this way. The ultraviolet light, however, also splits many of the organic molecules, making them more susceptible to oxidation by ozone. The specification teaches that the oxidation process is not instantaneous and, therefore, the effluent stream must be exposed to the ultraviolet source for a substantial period of time before being ejected into the atmosphere.
Another device that relies on ultraviolet radiation is shown in Japanese Application No. 08019379, published Jul. 22, 1997. The document describes a range hood with a self-cleaning function. A light coating of catalyst is formed on filters irradiated by a bank of ultraviolet lamps located in front of an exhaust aperture. Effluent streams past the catalyst and lamps and is oxidized by contact with the degraded and exposure to the light and ozone created by the light. Many patents have issued that describe similar systems employing photocatalysts.
One of the problems inherent in any system in which ozone, or any other agent, is relied upon for the treatment of an effluent stream, is insuring a uniform dwell-time or residence-time of every part of the effluent stream. Short circuiting by some of the effluent stream is a problem, since the flow moving from a narrow high velocity stream to a large-diameter slow moving stream must give up energy by generating energetic subflows. These can randomly crisscross a chamber causing some of the flow to short-circuit the chamber. A large number of baffles can be used to spread the flow, but to do so, requires a high pressure drop and the baffle elements interfere with the transmission of ultraviolet light to all parts of the treatment volume.